1. Field of the Invention
The present invention relates to a radiation generating apparatus which can be applied to non-destructive X-ray imaging or the like in a medical equipment field and an industrial equipment field, and to a radiation imaging apparatus using the same.
2. Description of the Related Art
Generally, a radiation generating tube accelerates electrons to be emitted from an electron emitting source by high voltage, and makes the accelerated electrons irradiate a target including a metal such as tungsten to make the target generate radiation such as X-rays. The radiation which has been generated at this time is emitted toward all directions. Japanese Patent Application Laid-Open No. 2007-265981 discloses a transmission-type radiation generating tube which has a shielding member arranged in an electron-incident side and a radiation-emitting side of the target, so as to shield radiation that heads toward directions other than a necessary direction. Such a transmission-type radiation generating tube does not need to cover the whole periphery of the radiation generating tube or a storage container for storing the radiation generating tube with a shielding member such as lead, and accordingly can achieve the reduction of the size and weight of the apparatus.
Incidentally, in order to generate radiation suitable for radiation imaging, a high energy electron beam needs to be emitted by applying a high voltage of 40 kV to 150 kV between the electron emitting source and the target. Because of this, a high potential difference of several tens kV or more results in being generated between the electron emitting source and the target, and between the radiation generating tube and the storage container. Japanese Patent Application Laid-Open No. 2007-80568 discloses such a structure that an insulating oil is filled between the radiation generating tube and the storage container 1, and further such a structure that an insulating member is arranged in the storage container 1, as a unit for securing voltage withstanding against the high voltage as described above.
In the above described transmission-type radiation generating tube, a middle-point grounding method is adopted as a voltage applying unit, and accordingly the size and weight of the radiation generating apparatus can be further reduced. Here, the middle-point ground method is a method of setting a potential with respect to the GND earth of the target at +(Va−α) [V], and a potential with respect to the GND earth of the electron emitting source at −α [V] (however, Va>α>0), respectively. The value of α is an arbitrary value within a range of Va>α>0, but generally is a value close to Va/2. When such a middle-point grounding method is adopted, the absolute value of the voltage with respect to the ground becomes small, and a creepage distance necessary for securing the voltage withstanding properties can be shortened. Accordingly, the size and weight of the apparatus can be reduced.
On the other hand, high potential difference is generated between the shielding member which has been electrically connected with the target and a storage container 1 which is generally grounded to become a ground potential, and accordingly an insulating liquid is exposed to an electric-field concentrated region located in the vicinity of the end of the shielding member. In such an insulating liquid in the electric-field concentrated environment, such a problem has occasionally occurred that voltage withstanding is lowered as in the case that an electric discharge occurs between the shielding member and the storage container 1 which is set at the ground potential. Furthermore, there is the case in which an electric charge is transferred to the end of the shielding member from the insulating liquid in the periphery due to such an action of the electric field concentration. In this case, depending on a driving condition of the radiation generating apparatus, the insulating liquid occasionally deteriorates because of the denaturation of a component constituting the insulating liquid due to this electric charge transfer and the voltage withstanding has been occasionally lowered.
Japanese Patent Application Laid-Open No. 2007-80568 discloses that a cylindrical insulating sleeve for insulating high voltage and a cylindrical shielding body for shielding scattered X-rays are arranged in the peripheral portion of an X-ray emitting port on the external side face of an X-ray tube bulb, and that the whole of the insulating sleeve and the shielding body are immersed in the insulating oil. However, though Japanese Patent Application Laid-Open No. 2007-80568 discloses the radiation generating apparatus of middle-point grounding, the potential of the radiation emitting port is an approximately ground potential, and such a problem is hard to occur that an electric discharge is generated between the radiation emitting port and a storage container which is set at the same ground potential as that of the radiation emitting port. Furthermore, Japanese Patent Application Laid-Open No. 2007-80568 does not disclose a special reason why the place of the insulating member to be arranged is selected, and does not give a special suggestion for solving the above described problem.
For this reason, an object of the present invention is to provide a radiation generating apparatus in which a radiation generating tube is immersed in an insulating liquid in the inside of the storage container, and which achieves the enhancement of voltage withstanding against high voltage and the reduction of the size and the weight, and to provide a radiation imaging apparatus using the same radiation generating apparatus.